Significant amounts of aluminum and magnesium alloys are being included in vehicle body architecture, especially in the passenger compartment safety cage, or “greenhouse,” as a result of the need to introduce more lightweight alloys with higher specific strengths and stiffness. Lightweight alloys frequently must be joined to high strength ferrous materials to meet design and regulatory requirements. Dissimilar metal joints (such as boron steel to 6xxx series aluminum) are now being specified in structures that are subject to specified safety standards.
Mechanical joints, such as rivets or flow drill screws may be used to join dissimilar materials but the strength, durability, and corrosion resistance of such joints does not match the properties of similar material welds.
Extrusions and hydro-formed parts are very attractive for the safety cage and specifically the roof rail Body-In-White (BIW) construction because they can achieve very high stiffness and offer much better material utilization compared to sheet metal parts of similar mild steel configurations with welded flanges. A major roadblock to broad implementation of extrusions and hydro-formed parts is the lack of affordable mass production joining methods to integrate these parts into BIW structures. Joining methods such a resistance welding, MIG welding, TIG welding, and spin stir friction welding generate heat may introduce dimensional distortion and may detrimentally impact the microstructure or material properties of the parts made of special heat treatable alloys.
Several different types of joining methods are currently available and may be categorized as one-sided or two-sided methods. One-sided joining methods are critical to the implementation of extrusion to extrusion joining because of access problems relating to the closed internal voids in some extrusions. One-sided joining methods such as flow drill screws add cost to the assemblies and are not well suited to high strength steel parts. Two-sided joining methods such as self-piercing rivets and clinch joints require access to the back side of a joint and are difficult to use in some applications where extrusions or tubular parts are joined.
The above problems and other problems are addressed by this disclosure as summarized below.